SEED DISTRIBUTION 169 
Evidently no kind of flowering plant actually increases at 
any such rate as has just been suggested, or it would soon 
crowd most others out of existence. The means by which the 
unlimited multiplication of any one 
species is prevented are lack of ex- 
tremely rapid and thorough means of 
disseminating the seeds, multiplication 
of the insect and plant enemies of the 
species (a factor which is often not 
very important), and over-crowding or 
competition with other plants of the 
same or of different species. 
161. Competition as a check on in- 
crease. No one can realize just what 
competition among plants means un- 
less he makes some careful out-of-door 
studies of plants growing under condi- 
tions of great overcrowding. A por- 
tion of a grainfield too thickly sown, 
a very weedy bit of garden soil left 
to itself for the whole growing season, 
or a piece of recently cleared forest 
in which coppice growth is starting 
from old stumps, or where seedling 
trees are springing up in great num- 
bers —any one of these will teach a 
most important lesson. The writer 
has found wild-black-cherry seedlings, 
to the number of more than 100 to 
the square foot, beginning to grow in 
the spring under a large wild-cherry 
tree. As the parent tree was in thrifty 
Fie. 150. Cockleburs 
This troublesome weed often 
grows along a path, where 
men carry the seeds back and 
forth in their clothing, and 
animals in their hair or fur 
condition and its top was nearly 30 feet in diameter, there 
might have been some 70,000 seedling cherries every year 
killed by crowding and shade under this one tree, although, 
in fact, there were never so many as this. 
